See those barely-visible vertical striations?
No, not those. Not the big fat white ones on the bottom of the joist. The ones you can barely see on the side of the joist.
[Click to enlarge]
The fat white lines I’ve boxed out in red are the remnants of the old plaster ceiling. The white lines are where plaster keys stuck up above the lath, the brown areas between are where the lath was nailed to the joists. Boring.
The vertical lines on the side that I’ve boxed in green, on the other hand, aren’t just stains. They actually have a small amount of relief, projecting slightly from the general face of the joist. They’re saw marks, and tell us something about the wood. Before the introduction of mechanical sawmills in the United States, lumber was cut using hand saws, often pit saws. Early mills copied the back-and-forth action of hand saws, switching over to circular saws in the mid-1800s. Pit saws tended to make slightly slanted parallel straight marks on the side of the beams; circular saws made (and sometimes still make) parallel arced marks on the beams; vertical mills made vertical parallel marks, like the ones in the picture. There’s a nice summary here.
In this case, we already knew that the building was constructed in the 1830s, so seeing the marks was confirmation. In other cases, seeing saw marks can help provide dates for the building.